The present invention relates to an intermediate layer of an electrophotographic photoconductor. More specifically, the present invention relates to organic pigments used for the intermediate layer.
Electrophotographic photoconductors (hereinafter simply referred to as "photoconductors") having a photoconductive layer on an electrically conductive substrate (hereinafter simply referred to as a "substrate") are well known. To achieve a desirable level of image quality, the photoconductors must maintain a stable charge potential, a stable residual potential and high sensitivity during repeated use. An intermediate layer is often disposed between the photoconductive layer and the substrate to produce a photoconductor having excellent charging properties. This intermediate layer prevents reduction of the charging potential, which is caused when charges having an polarity opposite to that of the charge potential are injected from the substrate into the photoconductive layer.
The resins used for the intermediate layer include resins of the cellulose family (Japanese Unexamined Laid Open Patent Application No. H02-238459); members of the poly(ether urethane) family (Japanese Unexamined Laid Open Patent Applications Nos. H02-115858 and H02-280170); melamine family (Japanese Unexamined Laid Open Patent Application No. H04-229666, and Japanese Examined Patent Applications Nos. H04-31576 and H04-31577); phenol family (Japanese Unexamined Laid Open Patent Application No. H03-48256); and polyamide family (Japanese Unexamined Laid Open Patent Applications Nos. H02-193152, H03-288157 and H04-31870).
However, charge potential reduction and residual potential rise may still occur in photoconductors having a conventional intermediate layer containing one of the above described resins. Charge potential reduction and residual potential rise in turn lead to image density reduction and greasing. Furthermore, the electrical resistance of a conventional intermediate layer increases when a photoconductor having such an intermediate layer is used repeatedly in a low temperature and low humidity environment, or when the intermediate layer is thickened to cover spots and defects in the substrate. This increase in electrical resistance of the intermediate layer leads to further residual potential rise and reduction in sensitivity.
To obtain a photoconductor having high sensitivity and low residual potential without the problems described above, the electrical resistance of the intermediate layer is often adjusted. Previously proposed means to adjust the electrical resistance of conventional intermediate layers include addition of a metal powder, such as Al powder or Ni powder, to a conductive pigment, such as indium oxide, tin oxide or carbon (Japanese Examined Patent Applications Nos. H01-51185, H02-48175 and H02-60177); addition of organometallic compounds (Japanese Examined Patent Application No. H03-4904 and the Japanese Unexamined Laid Open Patent Application No. H02-59767); and addition of a conductive organic polymer, such as polypyrrole or polyaniline (Japanese Unexamined Laid Open Patent Application No. H05-61234). However, in the case of the metal powders, it is difficult to uniformly disperse the mixtures of the metal powders with the conductive organic pigments. Uneven dispersion and aggregation of the conductive organic pigments can create a defective coating film. As for the organometallic compounds and the conductive polymers, these are not yet widely used, since there remain problems with the solubility of these organic components and stability of the coating liquids containing these organic components.